[R-390] Intermittent

[Flowertime01 at wmconnect.com]

Gary,

Remember, these receiver have worked for over 50 years. There is nothing 
that can go wrong with these receivers that has not gone wrong for someone 
before. Any problem can be found and fixed. Any problem can be found with a 
signal generator and volt meter. Its been done this way by thousands of GI for 
the last 50 years. You can do it also. If need be these receivers can be 
diagnosed one stage at a time and restored no matter how many problems have 
moved into the receiver.

Roy put you on the right path.

Having got past the easy things like dirty switches connectors and tube 
filaments, it is time to get serious and do some signal injection to isolate 
the problem area.

Does the receiver do this in both MGC mode and AGC mode? If only in AGC 
mode, you know its an AGC circuit problem (common). 

You have to ask your self a question here. Do I have a signal generator 
that puts out 455 Khz or not?  This is a yes or no question. Either answer is 
correct either answer will still let you get find the problem in your 
receiver and fix it. But a yes answer is much easier. Signal generator may be 
defined as any oscillator you can park on the bench next to your receiver. It 
need not have any real pedigreed.

If the answer is no signal generator worry not. I can still send you enough 
mail to get you through the problem. And believe their will be plenty of 
other support.

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Turn the receiver up on its side and watch the gas regulator tube (R390 has 
2, R390/A has one). see if the tube goes out when the signal dies. It is 
just a clue that something is leaking enough to pull the regulated B+ line 
down below the regulation voltage.

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If you have a Real R390 (you lucky Fellow) pull the Audio deck.

Turn the receiver up on end (audio deck end down) so you can swing the deck 
out on the bench and do some voltage checks on the filter caps in the audio 
deck. Measure the unregulated B+ and regulated B+ while the receiver works 
and again when the receiver quits. You may get some clues. But likely not. 
IF you have an R390 check the 47 Ohm resistors (4 ea ) under the audio deck 
in the 6080 regulator tubes. Also look under the power supply deck for 4ea 
more 47 ohm resistors. These things get burn and not fixed when a tube goes 
bad. Then some time later (like now) they start going open enough to drop the 
B+ out of range.

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Real most likely problem is a leaky cap. Think that what was once a nice 
stable circuit has had a component that has taken on a second nature. Haunted 
if you will. Defective for those of scientific bent. The bad parts reaches a 
critical voltage / temperature / state and then either charges to a higher 
than expected value or discharges to a lower than expected value and thus 
drives its related circuit out of proper conduction. I hate these problems. 
These type problems have driven some to shotgun whole decks and replace every 
suspect cap in sight. Other good decks have been parted into oblivion for 
acting this way.

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The solution is to divide and conquer. Using an isolation cap (.1, .01,001 
what ever ceramic cap is on hand) between the generator and receiver, inject 
about 150 uv of 455Khz CW into E210 of your R390. E211 of your R390/A. This 
will be the grid of the third mixer on the RF deck. Set the function switch 
to MGC. RF gain to MAX. BFO off. Hang a DC volt meter on the diode load. 
Adjust the generator for about minus 7 volts on the diode load. Remove the 
meter or set it to accept more than -30 volts DC. Remove your ear from the 
audio output. Turn the BFO on. Adjust the BFO for a reasonable tone. Set the IF 
band width to 8 or 16 Set the audio response to wide. Adjust the audio 
output for a reasonable level. Things will drift around. Decide if the generator 
or receiver band pass drifted apart or the receiver lost its signal.

Go make a pot of coffee and enjoy a cup. It could be the last time you will 
enjoy a beverage for several days. Then come back to the receiver.

With the AGC grounded in the MGC mode, you will find the IF and Audio deck 
either work as expected and out put a stable signal or the receiver looses 
it.

If the receiver looses it with this test (likely), you can start looking in 
the IF and audio deck. Big black plastic capacitors are suspect by 
association and age.

You can pick your way through the deck by injection and stage gain (best) 
Or by Ohm meter and capacitor checker. 

Please report back on the following. Toss in any thing else you find of 
interest and we can direct you down the path to a good solution. If thing look 
not good get verbose. Some of the Fellows like to read the gritty ugly 
stuff. They may even ask you for private pictures.

1. Real R390 or Real R390/A if you say non A and Les reads about it, your 
on your own.

2. Things under the power supply deck look good (an easy eyeball 
inspection).

3. Things under the Audio deck look good (an easy eyeball inspection)

4. B+ looks to be holding up OK and the gas tubes stay lit when the 
receiver looses its signals or they are going out.

5. The receiver does this in both AGC and MGC or only AGC or only MGC.

6. You have a pretty good to excellent signal generator on hand (you were 
able to do the test above) (yes or no)?

7. You have a DC volt meter on hand?

8 You have an AC volt meter on Hand (may be the same meter)?

9. Your luck and the AC volt meter has a DB scale? (not necessary) or you 
have a power meter (again not necessary).

10. You got a signal into the IF deck and it was stable or it did the same 
thing the receiver does when you put an antenna on the input?

Have fun with this.
Enjoy your self.
We can take it one on one if you want.
Stay with it, It may take weeks depending on how much time you have and the 
mail exchange.
But worry not this problem can be found and fixed.

Roger Ruszkowski AI4NI 

(Flowertime is the other hobby growing in the back yard under the antennas.)